Definition
In a woven aircraft fabric, the weft threads are the crosswise threads that run from selvage edge to selvage edge, perpendicular to the long warp threads that run the length of the bolt. Also called fill or filling threads.
Plain English
The threads that run side-to-side across a piece of woven fabric, crossing over and under the threads that run the long way.
Context Anchor
Seen in aircraft fabric covering, fabric inspection, and fabric repair discussions.
Derivation
Weft comes from the Old English wefan, meaning 'to weave.' The weft is literally what is woven across the warp. Knowing this helps separate the two: warp runs the length of the cloth (the long, tensioned threads on a loom), and weft is woven across them.
Why Pilots Care
Aircraft fabric has different strength along the warp than across the weft. Mechanics must orient the fabric correctly during covering and repair so loads are carried by the stronger direction, and so shrinkage during doping or heat-tautening behaves as expected.
Intuition Check
Do not read “weft threads” as just any threads in the fabric. In this context, they are specifically the threads that run across the width of the fabric.
Example Sentence 1
When laying out the fabric on the wing, the technician aligned the warp threads spanwise and the weft threads chordwise, as called out in the covering manual.
Example Sentence 2
During inspection, tears were found running parallel to the weft threads, indicating tension damage.